In such a system it is known to employ, at a switching terminal of a central office, a switch marker controlling a line concentrator/distributor which on an outgoing call connects the line of the calling subscriber to an available trunk or other inter-office link and on an incoming call connects a designated link to a selected local subscriber line; see, for example, U.S. pat. No. 3,328,534. The markers at the intercommunicating terminals are interconnected via a common signal path which carries the information relating to existing or desired connections between a given link and a local subscriber line. The line concentrator/distributor (simply referred to hereinafter as a line concentrator), inserted between n subscriber lines and m links where m is substantially smaller than n, comprises a multiplicity of switches whose selective closure thus allows up to m conversations to be carried out simultaneously. The marker, as a central component, communicates with the local lines and the inter-office links through the intermediary of peripheral units such as test circuits and couplers.
In a commonly owned application filed concurrently herewith by Franco De Marco and Gualtiero Rigo, Ser. No. 657,263, a circuit arrangement has been disclosed for checking the continuity of a connection established between two central-office terminals, by way of a selected link, for the exchange of messages between a calling and a called subscriber station. That system, operating with test signals of a characteristic frequency transmitted in opposite directions over respective link branches, also includes means for ascertaining the integrity of the connection established by either line concentrator between the link and the associated local line with the aid of a phantom circuit including the transmitting and the receiving branch of that line.
Such integrity checks are important in order to avoid the prolonged engagement of a link for a call which cannot go through, or which might distort the transmitted messages, because of some irregularity in the operation of the connectors of the line concentrator.
There exist, furthermore, various operational modes or conditions at a subscriber station which are unrelated to the functioning of the line connectors yet which ought to be taken into account in determining whether or not the call should proceed to the summoning of the called subscriber and the release of the centralized components serving to set up the connection. Thus, for example, a station may be out of service on account of the opening of a master switch manually operable by the subscriber or responding to a local power failure, e.g., in the case of an automatic data transceiver. Also, in the case of a called station, it may be important to determine whether the equipment happens to be off the hook so that the transmission of ringing current would be ineffectual.